In many modern computer installations, optical fibers are often used as a communication link to route data from one computer to another. Fiber optic cables are a preferred communication link because of their low loss and high immunity to external interference. With most fiber optic transmission schemes, a laser diode or other coherent light source generates light at a specific wavelength that is modulated according to a particular pattern of bits to be transmitted. The modulated light is coupled into an optical fiber to carry the light to a switch fabric that routes the light to an intended destination. In some cases, an optical fiber will carry only a single wavelength of light. In other cases (generally called wavelength division multiplexing or WDM) an optical fiber will carry multiple wavelengths of light each bearing its own data signal. Such optical fibers can carry data within a single building, between buildings, over a metropolitan area, or cross country and inter-continentally (so called long haul).
In complex networks, such as in server farms or data centers, the number of optical fibers required to connect all the servers and other electronic components can be substantial. For example, simply routing signals between buildings in a data center can require 20,000-40,000 separate optical fibers. Because the light generated in each of these fibers is produced from a separate light source, the power required to operate the communication system (including cooling systems required to regulate the heat generated by the communications electronics) is significant.